38 
PURIFICATION OF COBALT FROM ZAFFRE. 
undissolved, which contains so large a quantity of bismuth, 
as to render it worth preserving. The solution now con- 
tains cobalt, nickel, iron, copper, bismuth, and arsenic, and 
on passing sulphuretted hydrogen through it, copper, arse- 
nic, and bismuth are precipitated; after driving off the ex- 
cess of sulphuretted hydrogen by the addition of a little hydro- 
chloric acid and boiling, oxides of nickel, cobalt, and iron are 
precipitated by ammonia, an excess of which dissolves the 
first two, and iron may be removed by filtration. 
To the ammoniacal solution largely diluted with water, 
that has been well boiled to free it from air, potash is added, 
which precipitates nickel; it is then filtered, and sulphuretted 
hydrogen passed through it; this forms sulphuret of ammo- 
nium with the ammonia present, which, reacting on the 
oxide of cobalt, precipitates the sulphuret of cobalt. This, 
when well washed, may be treated with acids, to form any 
of the salts. 
I have tried this process several times, and find it to an- 
swer perfectly; a pure salt may thus be obtained at a very 
moderate cost, and a considerable quantity of bismuth as a 
bye product. The objections to the use of oxalic acid are 
that the nickel cannot be separated thus, as its oxalate is as 
insoluble as that of cobalt, and that the product being an 
insoluble salt, requires to be reduced with heat, in order to 
make the other salts. — lb. 
